This invention relates to rotary anode X-ray tubes in which a rotary anode is supported on a rotor which is supported by contactless magnetic bearings.
In the X-ray tubes, an anode emits X-rays as it is bombarded by high speed electrons from a cathode. The energy with which electrons bombard the anode is mostly converted to heat elevating the anode to a high temperature as high as more than 1,000 degrees centigrade. To avoid partial overheating and melting of a particular portion of the anode, the anode is rotated while the tube is in operation. The rotational frequency of such a rotary anode is as high as 18,000 to 20,000 r.p.m. For supporting the drive shaft of the rotary anode in such a high speed condition magnetic bearings are used. Various rotary anode X-ray tubes, which use contactless magnetic bearings for supporting a drive shaft of a rotary anode, are well known in the art and include the one disclosed in U.S. Pat. No. 4,167,671. In this disclosed X-ray tube, magnetic bearings are formed by permanent magnets which are magnetized in a direction parallel to the drive shaft of the rotary anode. The magnetic fluxes produced from these permanent magnets mostly proceed through a rotor along the drive shaft. The drive shaft is stably held in position in its axial direction by adjusting the magnetic fluxes generated therefrom. The drive shaft must also be stably held in position in directions at right angles to its axial direction. To stably hold the drive shaft in a direction normal to its axial direction by using a magnetic bearings, electromagnets are used independently of the permanent magnets. When external forces are applied to the rotor in radial directions at right angles to the axial direction, therefore, large currents have to be supplied to the electromagnets to stably hold the rotor in a balancing position. At any rate, with the prior art rotary anode X-ray tube the rotor can not be stably held in position, both in its axial direction and in a direction normal thereto, with the sole magnetic forces of permanent magnets. In some X-ray CT apparatus, however, an X-ray tube is rotated about a man's body. In this case, application of radial forces to the rotor in directions normal to the axis of the rotor is inevitable.
In the prior art rotary anode X-ray tube as described above, the magnetic path through which the magnetic flux produced from the permanent magnet passes, passes through the rotor to return to the magnet. The path includes comparatively large gaps, i.e., comparatively high magnetic reluctance portions. These gaps cause much flux leakage making it difficult to hold the rotor stably in the axial direction with only the permanent magnets. Comparatively large currents must be supplied to the electromagnets to make up for the insufficient force of the permanent magnets in order to hold stably the rotor in the axial direction.
In the above prior art rotary anode X-ray tube the permanent magnets and electromagnets are disposed on the outer side of a casing accommodating the rotor, which is undesirable when trying make the X-ray tube compact. The rotor is also sealed in the casing, and heat transferred to the rotor from the rotary anode is cooled only by high temperature radiation cooling from outside of the rotor reducing cooling efficiency.